Analytic Trigonometry

Transcript
Analytic Trigonometry

Analytic Trigonometry
Barnett Ziegler Bylean
Trigonometric functions
CHAPTER 2
recap
• In ch 1 we first defined angles – our way of
measuring them was based on a circle
• We then narrowed our focus to angles of a
triangle and explored similarity of triangles
• We finally zeroed in on right triangles and defined
and named 6 ratios- forming relations between
angles and these ratios that are functions
• These functions however, have a very limited
domain -
Degrees and radians
CH 2 – SECTION 1
Radian
• For various reasons the degree measurement
used with triangle trigonometry is cumbersome
and restrictive
• Therefore a new unit of measure was devised
• Definition:
•
1 radian = the angle which subtends an arc
that is 1 radius long
• Since a full circle is an arc of 360⁰ with an arc
length(circumference)
of 2π
•
360⁰= 2π radians
Conversion factor
• 360⁰= 2π radians yields conversion factors
• Examples: convert the following angle
measurements
• 20⁰
32⁰
120⁰
480⁰
•
2
3
rad
1 rad
2.46 rad
7.9 rad
Arc length/sector area revisited
•
•
since 1 radian subtends an arc with length of 1 radius
ө radian subtend an arc with length of ө radii
in other words s = өr hence =
2
2
2
2
• And
=
which becomes =
• Example: find the angle that subtends a 6cm arc on a circle with a 4
cm radius
• Example: find the arc length of an arc subtended by an angle of
radian 7 with a 9in radius
• Example : given a 22⁰ angle centered in a circle with 10 inch radius,
find the length of the arc it subtends
• Example: find the area of a sector enclosed by an angle of 1.7 rad
with a 4 in radius
The unit circle
CH2 – SECTION 3
Unit circle viewpoint of trig
• Sometimes looking at something from a different
viewpoint gives us useful information/tools with
which to answer various questions/problems
• Defining the trig functions by a triangle restricts
their use to angles 0 ⁰< ө⁰< 90⁰
• By using circles we have determined that angles
larger than this and smaller than this exist.
• We will now take a second look at our triangle
ratios
Addendum
• The equation for a circle is :
•
(x – h)2 + (y – k)2 = r2
•
where (h,k) is the point at the center of the
circle and r is the radius of the circle
Triangles and circles
For any point (x,y) on circle you
can draw an angle in standard
position where the terminal side
is part of a right triangle with
sides that are x any y long and a
hypotenuse that is r long.
This triangle is referred to as a
reference triangle. Its angle at
the origin is called a reference
angle
Through this triangle you can
associate the six trig ratios with
any point on the circle thus
expanding our domain to all
angles.
Using reference angles to find trig
ratios
•
•
•
•
•
•
Given a point (x1,y1) you can draw a circle centered at the origin
that crosses through the point and has a radius (r)
The equation for the circle is x2 + y2 = r2
where = 1 2 + 1 2
You can then draw an angle in standard position whose terminal
side goes through (x1,y1) and construct a reference triangle with
angle ө at the origin. Although length is usually thought of a
positive number we could attach a sign to the length in order to
further describe the reference triangle
The side opposite ө has length = y
The side adjacent to ө has length = x
And the hypotenuse of the triangle = r = x2 + y2
Using a point on a circle and a
reference triangle
• Let ө be the angle whose terminal side goes
through the given point and өr be the central
angle of the reference triangle
• A. (3,4)
B. (-4, 7)
c. (-2, - 6)
• D.
35 14
,
7
7
• There is a direct relation between the x
coordinate and cos(ө) and the y coordinate
and sin(ө).
Defining the trig ratios by the unit
circle
• given x2 + y2 = 1 (called the unit circle) then for any
point (x,y) on the circle :
cos(ө)= x
sin(ө) = y
sec(ө) = 1/x = 1/cos(ө)
csc(ө) = 1/y = 1/ sin(ө)
tan(ө) = y/x = sin(ө)/cos(ө)
cot(ө)= x/y = cos(ө)/ sin(ө)
• Note: this definition has done two things
1) it has expanded the domain of the functions
2) it has included negative values for the range
of the functions
NOTE
• We could have defined the trig ratios from a
generalized circle that has a radius of r.
• If we had then the definitions would read
cos(ө)=x/r
sin(ө) = y/r
sec(ө) = r/x csc(ө) = r/y
But tan(ө) and cot(ө) remain the same as
when defined by a unit circle • Therefore the above relations are true for
points not on the unit circle
Using the definitions to evaluate trig
functions
2
=
3
• Given sin(ө)
and that the angle
terminates in the 3rd quadrant :
find exact values for cos (ө) and tan(ө)
• given tan(ө)
1
=
5
and cos(ө) <0
find exact values for sin(ө) and sec(ө)
•
More on Evaluating trig functionsgiven ө
• With a calculator – The calculator will deal
with the negative values of both ө and f(ө)
• Be certain that you are set in the correct
input mode (degrees/radians)
• Examples: find sin(ө) cos(ө) tan(ө)
ө = {135⁰,
5
3
,
11
280⁰, ±
4
}
However, it estimates the irrational values
Special angles and basic identities
CHAPTER 2 – SECTION 5
Trig identities
• An identity is a variable equation that is known to be always true
• In algebra the property statements are identities ex.
Commutative property
x +y = y + x
• We have already alluded to several trig identities. Knowing them
sometimes saves time and energy and sometimes is crucial to
working the problem
• I find that understanding each set helps me to remember them –
you will need to learn them
• The textbook lists all pertinent trig identities on its front cover and
on a tear out pamphlet. Flash cards might aid you in learning them.
• Use of the pamphlet/cards/ or book will be highly limited on testsprobably mostly prohibited
Pythagorean identities
• Since x2 + y2 = 1 for our unit circle
• cos2(ө) + sin2(ө)= 1 for all values of ө
• Thus sin2(ө) = 1 – cos2(ө)
•
cos2(ө) = 1 – sin2(ө)
Fundamental trig. identities
• tan
• sec
• csc
sin()
= =
cos()
1
1
= =
cos()
1
1
= =
sin()
• cot =
=
cos()
sin()
=
1
tan()
• Thus once we determine cos() and sin() the
other six values are quickly determined
More on identities
•
We also noted earlier using triangles that complementary angles are related
cos(ө)= sin(90⁰- ө) and
sin(ө) = cos(90⁰- ө)
•
Written in radian notation
cos(ө) = sin( 2 − ө) and
•
Ex: cos( 32⁰) = sin(58⁰)
•
•
•
2
5
= cos
sin(ө) = cos( 2 − ө)
10
It is also well to note that full rotations ie ө and ө+ 360⁰(n)
or in radians
ө+2πn
are co-terminal angles thus have the same trig ratios
example: if sin(x) = .2981 then sin(x +14π) = .2981
And negative angle identities
• sin(-x) = - sin(x)
• cos(-x) = cos(x)
ө
-ө
Special angles
• Using some basic geometry there are some
angles whose trig values can easily be found
exactly even though they are irrational
• angles that are co-terminal with or reference
to: (ө) cos(ө) sin(ө) tan(ө) sec(ө) csc(ө) cot(ө)
0⁰ | 0
6
|4
|3
|2
30⁰ |
45⁰
60⁰
90⁰
Determine sign
• You can memorize the table
Ө values
Cos
sin
tan
sec
csc
cot
0 + 2πm < ө < π/2 + 2πm
pos
pos
pos
pos
pos
Pos
π/2 + 2πm < ө < π +2πm
neg
pos
neg
neg
Pos
Neg
π + 2πm < ө < 3π/2 + 2πm
neg
neg
pos
neg
neg
Pos
3π/2 + 2πm < ө < 2π +2πm
pos
neg
neg
pos
neg
neg
Ө = 0 + 2πm
1
0
0
1
undef
undef
Ө= π/2+ 2πm
0
1
undef
undef 1
0
Ө=π+ 2πm
-1
0
0
-1
undef
Ө= 3π/2+ 2πm
0
-1
undef
undef -1
undef
• Or utilize reasoning using quadrants and a unit circle sketch
0
Finding exact values for special angles
• Any angle that is a multiple of the special
angles listed in the previous table is either
co-terminal to or referenced by one of these
angles
• Find exact values for:
2
3
7
cos( )
6
cos(135⁰)
sin
7
3
sec(- 120⁰)
• Find the smallest positive angle such that
sin(ө)= -0.5